Hitherto, porous membranes of cellulose acetates, polyethylenes, polypropylenes, polymethylmethacrylates, polyacrylonitriles and polysulfones have been used in processes such as reverse osmosis, ultrafiltration, micro filtration and so forth. Unfortunately, however, these known porous membranes are still unsatisfactory because of disadvantageous heat-resistance and chemical-resistance properties, in particular anti-alkali, anti-acid and anti-solvent properties.
The specifications of the U.S. Pat. Nos. 3,953,566 and 4,096,227 disclose methods in which a non-sintered PTFE resin blend containing a liquid lubricant is extruded at a high temperature and the extruded material is drawn and heated to about 327.degree. C. or above, whereby a polytetrafluoroethylene resin porous membrane is formed. These methods, however, suffer from inferior performance of the product due to pore controllability of the porous structure or inferior membrane-formability which results in an impractically large thickness of the product membrane.
Accordingly, an object of the present invention is to provide a PTFE resin porous membrane which is free from these problems of the prior art, as well as a separator which makes use of this porous membrane and which exhibits superior heat- and chemical-resistance properties with an extremely small rate of eluation.